Formally Informal…

Are there regulations that spell out that the viva is relaxed and has a format which allows for variation?

Or is there an understanding among everyone involved that this is serious, but that the rules are agreed on by the “community” rather than someone at the top?

Which is it? Could it be both? Or neither? Does it matter?

I think what matters is that candidates check the regulations, find out about experiences and expectations, then think about what they need to do to meet them. The classification for how formal the viva is or isn’t is just one more piece of baggage, that doesn’t help unless we know what’s really involved.